With middlelower Pinghu and Baoshi formations in western slope zone and lower Huagang formation in western subsag in Xihu Sag, East China Sea Basin as the objectives, this study describes the macro and micro distribution characteristics of lowpermeable and nearly tighttight sandstone gas reservoirs using drilling data, confocal laser scanning microscope and scanning electron microscope. Based on tectonicsedimentary settings, stable carbon isotopic compositions of natural gas and analysis result of light hydrocarbon, this study carried out comprehensive analysis of genesis, origin and migrationaccumulation of tight sandstone gas using source rock simulation and fluid inclusion technology. The results indicate that nearly tighttight sandstone gas in two study regions is mainly coaltype gas. Tight sandstone gas in western subsag mainly comes from coalmeasure rocks in the Pinghu Formation, and dark sapropelic matter may indicate existence of minor amount of oiltype gas. In addition, part of natural gas may be slightly affected by coalmeasure rock of the lower Huagang Formation. Nearly tight sandstone gas in western slope zone can be divided into two groups: moderately mature gas from local source rocks and mature late stagehighly mature gas from source rocks in western subsag. The mature late stagehighly mature gas in western subsag vertically migrated and accumulated via faults at 39 ~0 Ma. Moderately mature gas was injected into reservoirs in western slope zone at 2.8~0 Ma. Meanwhile, the mature late stagehighly mature gas from western subsag migrated into western slope zone via ladderlike channel which consists of faults and deltaic front channel sand bar.